Surface & Coatings Technology 363 (2019), 66-74

https://doi.org/10.1016/j.surfcoat.2019.02.030

Abstract

Hydroxyapatite-containing titanium oxide coatings were generated on novel CP-Ti grade 4+ alloys by plasma electrolytic oxidation (PEO) process in two Ca- and P-containing electrolytes. The coatings were obtained under direct current (DC) mode and unipolar pulsed mode at 10 Hz. The effect of the electrolyte composition and the electrical parameters on the coating's performance was investigated. The coatings were characterized with respect to morphology, thickness, and phase formation by scanning electron microscopy (SEM/EDX) and X-ray diffraction. In both electrolytes hydroxyapatite-rich titania coatings could be formed in-situ. The amount of generated hydroxyapatite and perovskite phase depends on the electrolyte composition as well as on the electrical parameters. The coating's corrosion performance was tested in pH 7.0 Hank's solution + 0.1 M H₂O₂ by open circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Corrosion resistance of coatings generated under unipolar pulsing is increased significantly. Equivalent circuit modelling of EIS spectra indicates a hierarchical PEO coating structure revealing that the enhanced corrosion resistance originates mainly from the inner barrier layer.